Any OnStar system before '04 can't be reprogrammed. There was a 6-9 mo overlap where the digital and analog systems were sold together. Vehicles sold in that period can be brought up to date with a reprogram. Vehicles before that cannot be switched over to digital unless an updated brain is installed. The update was in the neighborhood of a few hundred dollars a couple years ago, not sure what it would cost now. The new brain is something like $5-600 and then whatever the dealer would charge to install and program.
For additional info, contact your dealer. They'll have a bank of VINs for vehicles that can be re-programmed. If your vehicle isn't in that group of VINs then the switchover will require new hardware.